To prevent the inner surface of tubes/pipes from corrosion, oxidation, erosion or wear, a functional coating is typically applied. The coating may be a layer of metallic or ceramic coating. For instance, to increase the erosion/corrosion resistance of gun barrels for tanks, Cr and Ta coatings (100-200 μm) may be deposited.
In order to increase the coating adhesion to a substrate, and hence the coating lifetime, it is useful to clean its surface before the deposition of a coating. One may consider chemical cleaning or plasma cleaning. Chemical cleaning is to remove the dirt, rust or oil/grease from the surface, and it may be performed before the substrate enters the vacuum deposition system. Plasma cleaning is typically designed to remove the surface oxide (“native oxide”) which exists on nearly all metal surfaces when they are exposed to air and other contaminants that cannot be easily removed using chemical cleaning.
Plasma cleaning is usually performed inside the vacuum system. After the plasma cleaning, deposition of the coating begins as soon as possible to minimize the surface re-oxidation. All the effort in developing cleaning procedures including the chemical and plasma cleaning and a suitable transition to deposition is to ensure the coating adhesion; therefore the coating functionality can be maintained. This invention discloses a plasma cleaning method by which tubes/pipes can be cleaned and therefore the coating adhesion is ensured. In the following, we use the example for cleaning tank gun barrels and magnetron sputter deposition of tantalum to describe the invention. However, the principle can be applied for coating other tubes/pipes.